You can get a nice Klipsch set for $150, but that's obviously above your range here.
Logitech makes good bang for the buck speakers (and literally too if you're talking the boomy subwoofers).
If you're doing mainly music, I'd suggest getting a 2.1 set rather than a 5.1 set.
You should be able to find a z-3 set for around $50.
If you want something that's going to allow you to shake things off shelves, Logitech also makes a more powerful set, the z-2300s for around $100.
I've also read that the JBL Creature IIs are a nice ser for music and are in your range. I personally think they're too odd looking, but if you like the looks, they might be a good pick.
There are a couple other sets by Altec Lansing etc. that would be worth looking at too.
If you can go to a Best Buy or Circuit City or something to listen to some of the offerings it would give you a little better idea of what you like.